Genetic Exchange in Prokaryotes.docx

**Genetic Exchange in Prokaryotes** ­­ **Sexual Reproduction** Exchange/recombination of genetic material Offspring with genetic diversity Genetically different from parental organism Maximized the probability of variation **Sexual Reproduction in Microbes** Have simpler mechanisms for genet...

**Genetic Exchange in Prokaryotes** ­­ **Sexual Reproduction** Exchange/recombination of genetic material Offspring with genetic diversity Genetically different from parental organism Maximized the probability of variation **Sexual Reproduction in Microbes** Have simpler mechanisms for genetic exchange (e.g.. cell-cell transfer, genetic uptake) may also produce gametes, some cases, entire microbial cells can act as gametes Most without distinct fertilization event **Sexual Reproduction in Prokaryotes** Transfer of donor DNA recipient - Substitution - Addition Donor DNA can persist if it's part of a replicon Recombination requires stable hybrid progeny Higher probability = participating organisms are from closely related species. **Significance of Sexual Reproduction** higher probability of mutations to occur in a microbe population (natural selection) Higher genetic variations = faster evolution E.g. of phenomena involving genomic rearrangements - AR plasmids - Flagellar phase variation - Antigenic variation **Horizontal Gene Transfer** Lateral gene transfer/non-vertical: i.e. parent to offspring. Core genome: set of genes encoding fundamental metabolic functions present in all taxon members Accessory genome: set of non-essential genes: encode traits associated with drug resistance, virulence, and degrade xenobiotic compounds **Transformation** Pieces of DNA from bacteria taken up directly from environment. Competence: ability of a bacteria to take up DNA and be transformed - Varies with physiological state - Could be induced using lab manipulations - Calcium uptake - Electroporation A diagram of a dna sequence Description automatically generated DNA fragments taken up Donor DNA aligns with complementary bases Recombination occurs  **Griffith's Experiment** Described the conversion of non-pathogenic pneumococcal bacteria to a virulent strain A diagram of a living organism Description automatically generated In 1928, British bacteriologist Frederick Griffith conducted a series of experiments using *Streptococcus pneumoniae* bacteria and mice. Griffith wasn\'t trying to identify the genetic material, but rather, trying to develop a vaccine against pneumonia. In his experiments, Griffith used two related strains of bacteria, known as R and S. **Transduction** Bacteriophages function as vectors to introduce donor DNA  **Abortive Transduction:** Transient expression of one or more donor genes without formation of recombinant progeny. **Complete transduction:** Characterized by production of stable recombinant genes that inherit donor genes and retain the ability to express them **Generalized Transduction** - Aberrant virulent phages - Contains bacteria genome instead of phage DNA A diagram of a cell division Description automatically generated **Specialized Transduction** - Mediated by temperate phages - Few specific donor genes can be transferred to recipient - Lysogenic -- lytic cycle release phage - Transducing phage - Contain part of bacterial genome adjacent to prophage attachment site  **Conjugation** Transfer of genetic material via direct contact. Donor ability is determined by specific fertility plasmids or sex plasmids. **F plasmid** - *E.coli* prototype for fertility plasmids in gram -- bacteria - F+: donors - F-: recipients - Conjugative functions **F plasmid conjugative functions** Specified by =\> 25 TRA genes - Expression F pili - Synthesis and transfer of DNA during mating - Interference of F+ bacteria as recipients F+ has 1 to 3 F pili that bind to specific outer membrane protein (ompA gene product) on recipient bacteria Cytoplasmic bridge formation 1 strand of F plasmid transferred to recipient Transferred strand -- circular ds F plasmid New strand synthesized in the donor strand A diagram of a cell division Description automatically generated Bacterial chromosome may integrate an F plasmid forming strains of bacteria called Hfr behave like F+ cells, but transfer whole chromosome instead of just F plasmid recipient cell may become F+ or recombine with chromosome and remain F- - i.e. DNA exchange but not F plasmid Conjugation can be used to map the location of genes in a bacteria chromosome greater distance from F origin = lower probability of donor gene getting into recipient  Needs to be circularized for replication transferred DNA must integrate with chromosome to remain *Q: You have an F- strain of Escherichia coli that is resistant to streptomycin but requires the following amino acids for growth on minimal medium: arginine, cysteine, methionine, phenylalanine and proline.* *You did a series of interrupted-mating experiments with one Hfr strain, which is wild type for all of the amino acid markers but are streptomycin sensitive. The following data are the times in minutes at which you first observed recombinant progeny carrying the indicated amino acid marker.* *Draw a chromosome map showing all the genes, the position of the Hfr and the direction of transfer The distances between the markers need only be approximate on your map.* A table with numbers and text Description automatically generated with medium confidence *Q: Interrupted mating was performed between an Hfr donor strain and a F-recipient with selection for his, leu, mal and xyl. The recipient was auxotrophic for all four genes. Following this interruption, recipients were scored for the rescue of auxotrophy: 90% were xyl+; 20% were his+; none were leu+, 80% were mal+. Draw a linear map to represent the distance between various markers and the origin of replication of the F factor in the Hfr strain.* **Recombination** Involved the breaking and joining of parental DNA molecules to form recombinant molecules  Enzymes - exonucleases - endonucleases - polymerases - ligases Happens between donor and recipient DNA molecules that have homologous nucleotide sequences. Nonhomologous aberrant recombination = illegitimate recombination **Transposons** DNA segments that can move from one site to other target sites Process: Transposition - Independent of generalized recombination - Cause mutations - Mediate genomic rearrangements - Portable regions of genetic homology - Acquire new genes Enzyme: Transposase Its insertion interrupts linear sequence of a gene and activates it - Deletions - Duplications - Inversions - Fusions between replicons Not self-replicating and must integrate into other replicons to be maintained in bacterial genomes **Transposition Types** **Non replicative:** "Cut and paste". Excision of the transposon from a donor site followed by its insertion into a target site is called nonreplicative **Replicative:** "Copy and Paste Transposon at a donor site is replicated and a copy is inserted into the target site. **Transposon Classes** **Class I** - Insertion Sequences -- transposase, no resolvase - Composite Transposons -- contains insertion sequences, encodes multiple functions not essential for transposition **Class II** - TnA Family - Longer terminal Inverted Repeats - Lack terminal insertion sequences - Encode both transposase and resolvase **Class II** - Bacteriophages MU and related temperate phages - Entire phage genome acts as transposon - Mutator phages **Class IV** - Discovered in Gram + bacteria - Consists of unique conjugative transposons - Tn917